The present invention relates to circuitry for amplifying signals transmitted through a transmission line and is directed more particularly to circuitry which automatically varies the amplification afforded to signals transmitted through a transmission line in accordance with the a-c losses of that line.
In communication systems wherein a-c signals are transmitted through transmission lines, it is often necessary to provide circuitry which compensates for the attenuation of those signals on transmission through the lines. In telephone systems, for example, it is necessary to provide amplifier circuits, or repeaters, to maintain satisfactory signal transmission through transmission lines which, in the absence of such circuits, would excessively attenuate the signals transmitted therethrough.
In some telephone systems it is necessary to compensate for frequency dependent losses, e.g., the frequency dependent attenuation of signals in a non-loaded transmission line. In these systems, a repeater provides a frequency dependent gain which combines with the frequency dependent attenuation of the transmission line so that the overall response of the system including the repeater and the transmission line is relatively flat over the voice frequency band. One such repeater is shown and described in U.S. Pat. No. 3,818,151 granted in the name of C. W. Chambers, Jr. et al. on June 18, 1974.
In other telephone systems, it is necessary to compensate for losses that are independent of frequency, e.g., the attenuation of signals in a loaded transmission line. In these systems, a repeater provides a relatively fixed gain over the voice frequency band. As a result, when the fixed gain repeater is connected to the transmission line, the system including the repeater and the transmission line decreases the attenuation of signals transmitted therethrough while retaining the desirable response characteristics of the transmission line. One such repeater is shown and described in U.S. Pat. No. 3,706,862 granted in the name of C. W. Chambers, Jr. on Dec. 19, 1972.
Due to the relatively high cost of purchasing and operating a separate repeater for each transmission line, it has been found desirable to operate repeaters in a common mode configuration, i.e., switch a relatively small number of repeaters among a relatively large number of occasionally used transmission lines. Such common mode systems may be of the type in which a repeater providing a frequency dependent gain is switched among several non-loaded transmission lines or may be of the type in which a repeater providing a fixed or frequency independent gain is switched among several loaded transmission lines.
A problem with switching a repeater which provides a frequency dependent gain among several non-loaded transmission lines is that the frequency dependent attenuation characteristic of a non-loaded transmission line varies as a function of the length of that line. As a result, a repeater which establishes a flat system response for a particular length of transmission line will be unable to establish a flat system response for a different length of transmission line. In order to overcome this problem, non-loaded transmission lines of similar gauges and lengths may be grouped together in a common mode configuration for servicing by a single frequency compensating repeater which is operable with any member of the group. Prior to the present invention, however, non-loaded transmission lines of differing lengths could not be grouped together in this manner even if they were of similar gauges.
A problem with switching a fixed gain repeater among several loaded transmission lines is that the attenuation of a loaded transmission line varies in proportion to the length of that line. As a result, a given repeater may establish a desirable system response (e.g., 6 db system loss) for a particular length of transmission line, but will be unable to establish that same level of system loss for a different length of transmission line. In order to overcome this problem, it has been the practice to group together in a common mode configuration loaded transmission lines of similar gauges and lengths and to utilize therewith a fixed gain repeater which is operable with any member of the group. Prior to the present invention, loaded transmission lines of differing lengths and gauges could not be grouped together and utilized in the common mode configuration, particularly if one or more of those lines was serviced by a d-c voltage boosting device commonly known as a loop extender. The most nearly satisfactory circuit of this type is shown and described in U.S. Pat. No. 3,851,108 granted in the name of Laimons Freimans on Nov. 26, 1974.
In accordance with a first embodiment of the present invention, there is provided circuitry which automatically varies the frequency dependent gain characteristics of a repeater in order to establish a flat system response for transmission lines of differing lengths, i.e., lines having differing frequency dependent attenuation characteristics. Similarly, in accordance with a second and a third embodiment of the present invention, there is provided circuitry which automatically varies the frequency independent gain of a repeater in order to establish substantially the same system loss for transmission lines of differing lengths and gauges.